This is a revision of R21 AT007222-01 to study the mechanism of action of hyperbaric oxygen (HBO2) in producing pain relief. Earlier studies demonstrated an antinociceptive effect of HBO2 in models of acute pain, including an unparalleled weeks-long antinociceptive effect, the development of which was prevented by inhibition of nitric oxide (NO) production at the time of HBO2 treatment. More recent studies have shown that HBO2 treatment produced an antiallodynic effect in rats with paclitaxel-induced neuropathic pain that lasted for up to four weeks following HBO2 treatment. The role of NO in the genesis of such a long-acting effect of HBO2 is unique and needs to be explored and exploited for clinical management of pain. Our long-term goal is to identify the pathway that is activated by HBO2 through NO and mediates a very long-lasting relief of pain. The objective in this R21 proposal is to determine the possible relationship between HBO2-induced changes in NO function and its antiallodynic effect. Our central hypothesis is that HBO2 stimulates the prolonged production of NO, which, in turn, activates a descending pathway involving opioid, GABA and 5HT1A mechanisms that may be responsible for an antiallodynic effect of unusually long duration. Our specific aims are to: 1) characterize the time course and underlying mechanisms of the antiallodynic response to single and repeated HBO2 treatments; and 2) characterize HBO2-induced changes in nNOS expression and determine the anatomical relationship between brainstem nNOS expression, GABA and descending serotonergic antinociceptive neurons. Pharmacological, immunohistochemical and molecular approaches will be employed in achieving these specific aims. The approach is innovative because it will elucidate the mechanism of analgesic action of HBO2, a treatment modality that has not typically been used for pain relief. The significance of the proposed research is that it will provide a scientific basis for clinical use of HBO2 in chronic pain management and identify the responsible neural pathway so that it may be targeted for small molecule development to activate the same mechanisms to produce extremely long-lasting relief of chronic pain.